Immobility time in 5d-FSSM. A, B, Six-week-old C57BL/6 mice were forced to swim on 5 successive days (D) for 10 min at ZT1 (A; n = 6) or ZT 14 (B; n = 6) following the advice of . Animals were then tested on days 12 and 26. The immobility time was measured over the 10 min of swimming, but only the first 4 min were analyzed. A, Repeated one-way ANOVA revealed an effect of time: *p < 0.01 vs day 1; post hoc Tukey–Kramer test showed a difference between D1 and D4. B, On day 40, mice (n = 6) received injections of 8-OHDPAT (1 mg/kg, i.p.), a prototypical 5-HT1A receptor agonist, 30 min before the session. Another group (striped bar, n = 6) did not receive 8-OHDPAT injection or undergo swim stress on day 40 but was forced to swim again on day 61. Repeated one-way ANOVA showed a main effect of time. **p < 0001, *p < 0.05 vs day 1, #p < 0.05 vs D61. Immobility time was significantly shorter on day 1 than on days 3–33. The injection of 8–OHDPAT significantly decreased the immobility time. Results are expressed as the mean values ± SEM.